A perpendicular magnetic recording scheme has been employed as a technique of achieving higher magnetic recording density. A perpendicular magnetic recording medium includes at least a non-magnetic substrate and a magnetic recording layer formed from a hard magnetic material. The perpendicular magnetic recording medium may optionally further include an underlayer that is formed from a soft magnetic material and performs a role of concentrating a magnetic flux generated by a magnetic head on the magnetic recording layer, an underlayer for orienting the hard magnetic material of the magnetic recording layer in a desired direction, a protective film for protecting the surface of the magnetic recording layer, and the like.
Japanese Patent Application Publication No. 2001-291230, Japanese Patent Application Publication No. H08-083418, and WO 2002/039433 disclose granular magnetic materials as a material for forming the magnetic recording layer of the perpendicular magnetic recording medium (see Patent Documents 1 to 3 and the like). The granular magnetic material includes magnetic crystal grains and a non-magnetic material segregated so as to surround the magnetic crystal grains. The individual magnetic crystal grains in the granular magnetic material are magnetically separated by the non-magnetic material.
In recent years, in order to further improve recording density of a perpendicular magnetic recording medium, there is a need to reduce the size of the magnetic crystal grains in the granular magnetic material. On the other hand, a reduction in the size of the magnetic crystal grains results in deterioration in the thermal stability of recorded magnetizations (signals). Thus, in order to compensate for the deterioration in the thermal stability caused by the reduction in the size of the magnetic crystal grains, it is requested to form the magnetic crystal grains in the granular magnetic material using a material having higher crystalline magnetic anisotropy.
Moreover, in a magnetic recording layer formed from a granular magnetic material, there is a problem that a plurality of magnetic crystal grains forms (clusters) one magnetization reversal unit. This problem is considered to result from an exchange interaction between magnetic crystal grains. In order to solve this problem, Japanese Patent Application Publication No. 2011-119006 discloses a magnetic recording layer in which a first layer that contains Ru, Cr, Ti, Ir, or oxides of these metals as a non-magnetic material and has negative exchange interaction and a second layer that contains SiO2 as a non-magnetic material and has positive exchange interaction are stacked, whereby exchange interaction is suppressed (see Patent Document 4).
A L10-based ordered alloy is proposed as a material having the required high crystalline magnetic anisotropy. Japanese Patent No. 3318204, Japanese Patent No. 3010156, Japanese Patent Application Publication No. 2001-101645, Japanese Patent Application Publication No. 2004-178753, and Japanese Translation of PCT Application No. 2010-503139 disclose an alloy such as FePt, CoPt, FePd, or CoPd that includes at least one element selected from the group consisting of Fe, Co, and Ni, and at least one element selected from the group consisting of Pt, Pd, Au, and Ir as a L10-based ordered alloy (see Patent Documents 5 to 9). Further, these documents disclose various methods for manufacturing a thin film of L10-based ordered alloys (see Patent Documents 5 to 9).
On the other hand, since the thickness of a magnetic recording layer is basically uniform in an in-plane direction of a medium (the in-plane direction may be a direction which is in a lateral direction of the medium or perpendicular to a thickness direction of the medium), a reduction in the magnetization reversal unit (magnetic crystal grain) means a decrease in the cross-sectional area of the magnetization reversal unit (magnetic crystal grain) having a constant height. As a result, since a demagnetizing field acting on the magnetization reversal unit (magnetic crystal grain) decreases, and a required magnetic field (reversing magnetic field) for reversing the magnetization of the magnetization reversal unit (magnetic crystal grain) increases. Considering the shape of the magnetization reversal unit (magnetic crystal grain), it means that an improvement in the recording density requires a higher magnetic field during recording (writing) of signals (magnetizations).
In order to increase the recording magnetic field (writing performance), a heat-assisted magnetic recording scheme which uses a head having the function of heating a magnetic recording layer is proposed. This scheme utilizes temperature dependency on a magnetic anisotropy constant (Ku), of a magnetic material (that is, such a characteristic that the higher the temperature, the smaller the magnetic anisotropy constant Ku). That is, the temperature of the magnetic recording layer is increased to temporarily decrease the magnetic anisotropy constant Ku to reduce a reversing magnetic field, and writing is performed during this period. Since the magnetic anisotropy constant Ku returns to its original high value after the temperature decreases, it is possible to stably hold recording signals (magnetizations). When the heat-assisted magnetic recording scheme is applied, it is necessary to design the magnetic recording layer taking the conventional design guidelines and the temperature characteristics into consideration.
According to a study by Igarashi et al. “Computer Simulation for Thermal Assist Recording—The Recording Format” (Technical Report of The Institute of Electronics, Information, and Communication Engineers (IEICE), 2004, MR2004-39), the transition width between recording bits in the heat-assisted magnetic recording scheme is determined by a head magnetic field gradient and a temperature gradient (see Non-Patent Document 1).    Patent Document 1: Japanese Patent Application Publication No. 2001-291230    Patent Document 2: Japanese Patent Application Publication No. H08-083418    Patent Document 3: WO 2002/039433    Patent Document 4: Japanese Patent Application Publication No. 2011-119006    Patent Document 5: Japanese Patent No. 3318204    Patent Document 6: Japanese Patent No. 3010156    Patent Document 7: Japanese Patent Application Publication No. 2001-101645    Patent Document 8: Japanese Patent Application Publication No. 2004-178753    Patent Document 9: Japanese Translation of PCT Application No. 2010-503139    Non-Patent Document 1: Igarashi et al. “Computer Simulation for Thermal Assist Recording—The Recording Format” (Technical Report of The Institute of Electronics, Information, and Communication Engineers (IEICE), 2004, MR2004-39